Sensors are known which take the form of an electrical discharge device for receiving a sample of an atmosphere suspected of containing a concentration of a gas to be detected and comprising cathode and anode elements for producing and collecting ions. The collected ions produce from one of these electrodes a current which varies with the concentration of the gas to be detected. One common use of such electrical discharge devices is as sensors within halogen leak detectors to detect the leakage of halogens and their gas compounds.
Many prior art halogen leak, ion detectors are portable and are carried by an operator to particular site by an operator where a leak of a relatively high level of concentration of halogens is suspected to be present. The operator turns on the leak detector to initiate the detection process. If a gas or halogen leak is detected, the operator typically deactivates the leak detector and proceeds to determine the source of the leak. Halogens are commonly used as refrigerants in domestic and relatively large, commercial refrigeration systems. The leakage of halogen refrigerants has been recognized as a danger to the environment and, in particular, to the depletion of the ozone layer. Taxes have been imposed on the sale of such halogen refrigerant gas, thus giving a financial incentive to closely monitor and prevent the loss of expensive halogen gases.
The halogen sensors in the form of such an electrical discharge device are relatively expensive. The life of such sensors is relatively limited requiring frequent replacement. Experience with that halogen sensor manufactured by Yokagowa Corp. under model No. 6614K11G1 has shown it to have a life of approximately 1500-1800 hours. The collector/cathode elements of such electrical discharge devices are coated with rare earth metals and tend to deteriorate rapidly during sustained collection of the ion stream, as occurs in the presence of halogen gases. In applications wherein such a halogen sensor and its leak detector are used to monitor halogen leaks within a given enclosure such as a refrigeration systems for relatively long periods of time, a halogen leak may occur and is accordingly sensed by the halogen sensor for a relatively long period of time until an operator may intervene to re-set, re-zero or otherwise disable the halogen sensor. During such delay before resetting, the collector/cathode element of the sensor is continually bombarded by the increasing level of ions due to the presence of the halogen. Such extended use quickens the deterioration of the collector/cathode elements and therefore the life of such halogen sensors.
U.S. Pat. No. 4,910,463 of Williams et. al, assigned to the assignee of this invention, disclose a halogen monitoring apparatus capable of monitoring for the presence or leakage of a gas and in particular, halogen, within a given enclosure such as a large commercial refrigeration system. This monitoring apparatus permits extended monitoring of halogen leaks and overcame the problem of the destruction of ion gas detectors by sensing for an increase in the output of the gas sensor and after a fixed, leak wait period deactuating the gas sensor. This gas monitoring apparatus employ illustratively that halogen sensor, which is manufactured by Yokagowa Corporation and described above. The ion stream drawn to its collector/cathode element provide a current of a magnitude proportional to the concentration level of the detected halogen. In particular, this halogen monitoring apparatus senses an increase in the collector/cathode current of a given magnitude and, after a fixed period, reduces the voltage applied between the heater/anode element and the cathode/collector to extinguish the current flow therebetween. Thus such a halogen sensor could be repeatedly used to detect the presence or leakage of a halogen gas, without destroying its collector/cathode element and the resultant shortening of the life of such halogen sensors. Also upon sensing of a cathode/collector current, this halogen monitoring apparatus would provide an alarm to prompt an operator to find and stop the halogen leak.
The Williams et al. Patent '463 avoids issuing false alarms due to the transient presence of the halogens to be detected. If a halogen is present only for a brief time, it is not desired to actuate an alarm, which would bring an operator to fix a gas leak. Williams et al. teaches that a fixed leak wait period is timed out before actuating the alarm. A completed leak wait period of Williams et al. includes three cycles; during each cycle, the gas sensor is deactuated for a first fixed period, before starting a second warm-up period. After the warm-up period, the halogen sensor is again turned on, and if a halogen concentration is detected above the predetermined limit, a second cycle of the wait period commences. William et al. suggest that 3 leak detections be counted, before issuing a valid leak alarm. The total time required for the 3 cycles to run is fixed, before a valid alarm leak is actuated. Further, if the concentration level of the gas detected falls below the set point, the timing process is restarted from the beginning. This has the disadvantage where the gas being monitored is of a concentration level substantially equal to the set point. In those situations, the process of timing 3 cycles may be repeatedly initiated when the sensed level of gas concentration momentarily falls below the set point, thus restarting the recycle process any number of times and thus delaying the actuation of the alarm. Further, the use of a fixed leak wait period does not adequately protect the gas sensor when relatively large concentrations of the halogen detector are detected. Higher ion currents drawn to the collector/cathode element tend to more quickly destroy that element.
U.S. Pat. No. 4,297,689 of Shaw et al. is illustratively of those gas measuring devices, which sense a gas concentration level above a threshold value to actuate an alarm after a variable wait period inversely dependent upon the concentration of the detected gas. However, such gas detecting apparatus do not teach that the sensors are ion-type gas detectors or that the detector is deactuated after a variable period to prolong its life.
Further, the Williams et al. patent '463 describes a circuit for energizing and regulating the temperature of a heater/anode element of its gas sensor, which is more fully described in U.S. Pat. No. 3,912,967 of Longenecker. Longenecker senses the resistance of the heater/anode element and closes a transistor switch coupled to the heater/anode element, when the heater/anode resistance is below a threshold value and opens the switch when the heater/anode resistance exceeds that value. It is important to regulate the temperature of the gas sensor because a change in the temperature results in a change of the background current level, which may be erroneously attributed to the presence of halogen gas. Longenecker provides a coarse control of his heater/anode element temperature by comparing the element's resistance and therefore its temperature to a desired resistance. If less, the transistor switch is disposed from its non-conductive to its conductive state thereby causing an increased current flow through its heater/anode element. The increased current causes the resistance and therefore the temperature of the heater/anode element to rise until it is equal to the desired temperature. When equal, the transistor switch is rendered non-conductive, whereby the current to the heater/anode is reduced. The Longenecker controls the transistor and thus the heater/anode element temperature in an on/off fashion, whereby an increased or decreased current is applied thereto. Longenecker does not teach a proportional control as would provide a finer, more accurate control of the heater/anode temperature.